The present inventor previously invented the Stretched Fresnel Lens for Space Power, as described in U.S. Pat. No. 6,075,200 (Reference 1). NASA and the U.S. Department of Defense have recognized the many advantages of this Stretched Lens Array (SLA) technology for space power, and have funded a number of R&D contracts totaling about $10,000,000 to develop, test, mature, and fly SLA technology (References 2 and 3). SLA offers dramatic improvements in all of the critical performance metrics for space solar arrays, including lower cost ($/Watt of array power output), higher specific power (Watts/kilogram of array mass), higher stowed power (Watts/cubic meter of launch volume), improved photovoltaic cell radiation hardness (due to smaller cell sizes, allowing thicker radiation shielding at lower mass penalty), and higher voltage operation (again due to the smaller cell sizes, allowing thicker dielectric insulation at lower mass penalty).
However, the previously patented version of SLA is substantially limited to relatively small (e.g., <15 degrees) longitudinal incidence angles of sunlight, typically called beta (β) angles in the space solar array industry. For many missions, however, the longitudinal incidence angles (β angles) can be relatively large, and the existing SLA technology will not work well for such missions. For example, NASA is planning missions which will use solar electric propulsion (SEP) to transport spacecraft from low earth (LEO) orbit to higher orbit. For such missions, it is often desirable to use a non-equatorial launch site such as Cape Canaveral in Florida, resulting in a relatively large initial orbit inclination angle (e.g., 28 degrees). This large orbit inclination angle, coupled with seasonal variations in the sun-earth declination angle (±23.5 degrees) can resulting in longitudinal incidence angle (β angles) of about 51 degrees (28 degrees+23 degrees) for a single-axis sun-tracking solar array. The existing SLA will not provide an acceptable focus of sunlight for such large β angles. A new type of SLA is needed for such missions.
New multi-junction solar cells are being developed with four junctions and six junctions to improve the solar-to-electric conversion efficiency, but such cells are very expensive with projected costs of more than $10 per square centimeter of cell area. To enable near-term cost-effective use of such sophisticated solar cells, it is highly desirable to provide at least 4× or higher concentration ratio (lens aperture width divided by cell active width), to reduce the cell area and cost by about 75%. The cell efficiency also increases with concentration ratio, offsetting the lens transmittance loss, and making the combination of lens and cell even more cost-effective relative to one-sun planar arrays using the same cell technology.
The present invention is a new line-focus Fresnel lens solar concentrator which is configured to focus sunlight at large longitudinal incidence angles (β angles) by using a unique prismatic geometry in combination with an articulating energy receiver to maintain a relatively high concentration ratio.